The Japanese population routinely consumes iodine-rich seaweed, thereby probably making Japan the nation with the highest iodine intake worldwide. The present study aimed to estimate the duration of dietary records (DRs) needed to calculate the usual iodine intake and to ascertain the frequency of iodine intakes above the tolerable upper intake level (UL) in the Dietary Reference Intakes for Japanese. Four 3-d DRs for the 4 seasons within a year were collected for 55 men and 58 women. On the basis of analysis of variance, the total variance in iodine intake was classified into inter-individual and intra-individual components. The frequency of appearance for high iodine intakes was estimated. The most commonly consumed types of iodine-containing food items were seaweed, milk and milk products, fish and shellfish, and tofu. The percentage contribution of intra-individual variance was markedly greater than that of inter-individual variance, and the excessive iodine intake was intermittent rather than continuous. The duration for which dietary records were required to assess the usual intake of iodine within 10% of their true mean was 6,276 d for men and 4,953 d for women. The period that transpired until a value was exceeded once was 6.3 d/occurrence for values above UL (2,200 μg), 8.5 d/occurrence for values above 3,000 μg, 9.8 d/occurrence for values above 4,000 μg, 11.2 d/occurrence for values above 5,000 μg, and 16.7 d/occurrence for values above 10,000 μg. To avoid errors in interpretation, it is inappropriate to assess the habitual nutrient intake of a nutrient that is intermittently consumed at maximal levels. It is important to assess the iodine intake in consideration of the range of the nutrient intake and of the time period in which the upper limit is exceeded.